This invention relates to telephone switching systems and to high-speed data communications, and, more specifically to a line card for analog telephone service that is compatible with high-speed data service on the same line.
Once upon a time, the Bell System wired almost every home and business in the United States for telephone service by one (or more) tip-ring pair of copper wires. This original physical plant provided plain old telephone service (xe2x80x9cPOTSxe2x80x9d). As time passed, more and more features and services were offered over the same copper wire. The tip-ring pair was originally intended for voice-band service of between (approximately) 0 and 4 kHz. This rather narrow bandwidth (by today""s standards) provides high-quality voice communication and high quality in-band communication in general, i.e., dual tone multifrequency (DTMF) signaling. In more recent times, however, home and business use of computers (and data transmission in general) has taxed this tip-ring infrastructure to the maximum amount of data that can be passed through the voiceband (approximately 53 kbps). Consumers, especially business consumers, are demanding greater data rates. Several new technologies have been tried in an attempt to fulfill this demand. Coaxial cable and optical fiber have been tried, both of which have much higher bandwidth than copper wire. The one major drawback to both is that, unlike the copper wire infrastructure, there is no universal cable or optical infrastructure.
One relatively new technology that can be implemented over the embedded copper networks is digital subscriber line (DSL). DSL comes in many varieties such as asymmetrical DSL (ADSL) (upstream and downstream have different bandwidth requirements) and other varieties of service (herein XDSL). Many XDSL technologies provide high speed data service over current tip-ring pairs by encoding the signals in frequency ranges above the POTS frequency. Thus, one tip-ring pair can provide both POTS service and high-speed data service.
The local service providers (who are no longer part of a universal Bell System), however, have to make major changes in their central office to provide XDSL service. FIG. 1 is a block diagram of a current central office providing XDSL service. Central office 10 includes a local telephone switching system (switch) 12 that provides the usual POTS telephone services and features. In order to provide XDSL, central office 10 also includes digital subscriber line access multiplexer (DSLAM) 14 which sends and receives digital signals to and from the subscribers and intefaces to the digital network 16. A main distribution frame 20, as is known in the art, connects outside plant tip-ring pairs 22 to switch 12 and DSLAM 16.
In order to serve both a POTS telephone and XDSL on one tip-ring pair 22, a splitter 24 is required. Splitter 24, in general, causes the XDSL signal to be sent towards the DSLAM 16 and the POTS signal to be sent towards local switching system 12. More specifically, splitter 24 includes a 4th to 6th order low pass filter 28. A connection 30 is made to the incoming tip-ring pair and rerouted back through MDF 20 to DSLAM 16 at XDSL line card 32 for providing XDSL services. The POTS connection passes through the low pass filter 28 and then back through MDF 20 to POTS line card 36 in local switching system 12. Splitter 24, and more specifically low pass filter 28, are required in order to filter the high frequency signal used by XDSL from the POTS circuit so that POTS line card 36 may process the incoming (and outgoing) signal properly. Furthermore, and more importantly, XDSL has a higher voltage (approximately 18 volts peak to peak) as opposed to POTS 3 to 5 volts peak to peak. Therefore, the low pass filter is also required to reduce the voltage sent to POTS line card 36 so that the line card is not damaged due to over voltage. The opposite is not true of XDSL line card 32, in that the POTS communication is so far outside of its frequency band and of such low voltage that such signals require minimal filtering and are taken care of by normal XDSL analog interfaces.
Several issues arise when splitter 24 is introduced into central office 10. Splitter 24 takes up office space, which is increasingly at a premium as new DSLAMs 16 and other services are incorporated into central office 10. Furthermore, wiring in the office becomes more complex. Wherein an original POTS line passed through MDF 20 directly to POTS line card 36, in order to provide XDSL service, the wiring passes through MDF at least three times for distribution of the various signals. In some instances, splitter 24 may be a part of the DSLAM 16 and separate wires run to switch 12 in some cases bypassing MDF 20. This scenario, however, further complicates the wiring distribution in the office and is generally avoided. In either example, if the POTS line card 36 or the XDSL line card 32 (and switch 12 and DSLAM 16 respectively) malfunction, it may affect the other equipment, and the attached subscriber can use neither the POTS telephone nor the DSL service.
In U.S. patent application Ser. No. 08/767,138, entitled xe2x80x9cTelecommunication Equipment Support Of High Speed Data Servicesxe2x80x9d which is assigned to the assignee of this invention, a new line card is disclosed that incorporates both POTS and DSL service. This is a superior solution to the above-described problem provided that switch 12 switches both POTS and XDSL service. Furthermore, many data service providers that are not the incumbent POTS service provider want to provide XDSL service. This requires that a DSLAM of the alternate service provider be connected to the switch in some manner, such as that shown in FIG. 1.
Therefore, there is a need in the art for a system that does not require additional equipment in order to provide XDSL service over POTS telephone lines.
This problem is solved and a technical advance is achieved in the art by an improved plain old telephone service (POTS) line card that may be directly connected to a customer line that is also connected to a digital subscriber line access multiplexer. A second order filter is added to the front end of the POTS line card in order to attenuate XDSL signals and to lower its amplitude. A new digital signal processor (DSP) includes further low pass filtering functions, gain compensation and termination impedance compensation to provide the proper balanced network as is known in current POTS cards but not provided in current XDSL services. This card may also be used alone without a XDSL equipped line. Thus, if the line is XDSL equipped maintenance may be performed on one without affecting the other of the two services. Further, these cards may be used in areas wherein DSL service is likely to be added as a feature and/or in areas with alternative XDSL providers.